An immunological assay for bacteriophage structural proteins is being developed for use with extracts from T4-infected cells to study the regulation of the synthesis of these proteins produced during the late stages of virus development. These extracts contain a DNase-sensitive component required for late T4 protein synthesis when combined with a cell-free preparation from uninfected cells. The T4 extracts will be fractionated using sucrose density gradients to determine whether the synthesis of late proteins requires substances other than DNA from the T4-infected cells. The relationship of DNA synthesis to late transcription determined by others using mutant phage will be tested in the in vitro system using DNA extracts from cells infected with the same mutants to see if the mutants can program the synthesis of T4 structural proteins.